1. Technical Field
The field addressed concerns sub-micron imaging--imaging based on least dimensions of less than 0.3 .mu.m. A primary objective is lithographic imaging in the fabrication of Large Scale Integrated circuits (LSI).
Processes of the invention depend upon excitation of electrons from a patterned photocathode by use of electromagnetic radiation--generally radiation in the ultraviolet spectrum (of energies greater than 5 eV).
2. Description of the Prior Art
It is widely recognized that continuing miniaturization of LSI from presently used 1.0-0.9 design rules will soon require a different approach to pattern delineation. Presently used radiation in the near-UV spectrum (.lambda.=0.4-0.5 .mu.m) is considered wavelength-limiting. As now practiced, it is the general view that deep-UV will be substituted, in turn to be limiting at =0.25 .mu.m design rule.
Considerable work is directed to extension of UV to design rules thought too small by the conventional view. Progress has been made on development and use of phase masks. The principle is phase cancellation of edge-scattered radiation. Development of this approach is now at an advanced level to accommodate variation in reference phase, e.g. due to proximity effects for closely spaced features, while continuing to produce the 180.degree. phase shift. It is expected that use of phase masking will permit extension to design rules perhaps half as great as the assumed wavelength limit.
The realization that a different form of radiation will be required for succeeding LSI generations--likely at the 256 mbit chip level--is responsible for a high level of activity worldwide.
World activity for the most part entails use of electromagnetic radiation of shorter wavelength--of soft x-ray. X-ray projection has proceeded apace with experimental systems producing satisfactory 0.1 .mu.m patterns (see, J. E. Bjorkholm, et al, J. Vac. Sci. Tech., v. BS, p. 1509 (1990)).
Electron imaging has not been overlooked. Evolution of a mask projection system gained from experience in the direct write Electron Beam Exposure System (see, "E-beam Technology", Brewer, ed., chapter by J. Trotel and B. Fay, pp. 325-335 (1980). Scattering with Angular Limitation Projection Electron-beam Lithography has been demonstrated in the form of reduction projection to yield a 0.1 .mu.m design rule pattern. (See S. D. Berger et al. J. Vac. Sci. Technol., B9 (6), p. 2996 (1991).)
Despite the obvious advantages of reduction projection--regarding cost and fabrication of the mask--considerable effort is directed to 1:1 x-ray imaging (see, G. K. Celler, et al, App. Phys. Lett., v. 59, p. 3105 (1991)). There are many who believe that increased mask cost associated with 1:1 is offset by design simplification and reduced apparatus cost relative to reduction projection. Experimental systems have demonstrated feasibility of mask construction and of proximity printing at 0.2 .mu.m design rule and smaller. Some believe that this approach will be only a last resort. Mask fragility, always an issue, is aggravated by the very close proximity required between mask and wafer. A major field of endeavor is directed to mask repair. The x-ray source continues to be a problem--for the most part effort is based on a synchrotron source, likely at a cost of at least ten million dollars.
A now-abandoned effort of some years ago was based on use of a photocathode. In its most advanced form, the photocathode, in replacing the mask, was constituted of a UV-transparent substrate, typically quartz, having one surface coated by a patterned layer of a good photo emitter, for example, cesium iodide, CsI. The "blocking" regions were constituted of a poor photo-emitting material, typically chromium (see, J. Vac. Sci. Technol. cited above). The structure was operated by illuminating the back surface with UV radiation (see, Trotel and Fay cited above).
Abandonment must be attributed to a number of drawbacks, some regarded as inherent, some perhaps due to limitations in then-available conditions and materials. For one thing the photocathode itself was regarded as short-lived--as needing frequent replacement. Recognizing that short life was likely due to contamination--to reaction despite use of the best vacuum practically available--an effort was apparently made to seek out less reactive material. Trotel and Fay at page 330 report the use of a noble metal--specifically of gold. The photocathode took the form of a very thin layer--of a 50 .ANG. thick layer--to optimize needed surface emission from the highly absorbing gold layer. The effort was unsuccessful--the cathode did not last long in operation and the approach was abandoned.
Problems associated with design of the photocathode projection system are discussed in IEEE Transactions on Electron Devices, vol. ED-22, No. 7, (July 1975). First characterizing such systems as having excessive image distortion, the author goes on to reach the conclusion that structural elevational differences on the anode--on the image plane--may be significant. Numerically, the conclusion is reached that achievement of .+-.0.3 .mu.m accuracy requires anode flatness of .+-.2 .mu.m. Attainment of smaller design rule patterns would presumably require greater flatness.
A more recent article, J. Vac. Sci. Technol., B4(1), pp. 89-93, (Jan/Feb 1986), addresses future needs. On the assumption that pattern area will be limiting, the authors first dismiss x-ray alignment registration marks as inadequate--presumably as unduly space consuming--and then suggest a step-and-repeat system depending on back-scatter alignment marks. Acknowledging that the detection system will not function in the presence of the high electric field required for resolution (for sufficiently short deBroglie wavelength), provision is made for a field-free region. Field-elimination is the consequence of a movable grid placed at the first magnetic focus point--at the distance corresponding with the first 360.degree. cyclotron period. Of course, the presence of the grid, while of minimal consequence as so positioned, continues to be an impediment and, in turn, requires significant apparatus/process complexity to avoid distortion of the image by the grid.